In nuclear fusion reactors, beryllium is considered a promising plasma-resistant material due to its excellent heat resistance and suppressed absorption of tritium. However, beryllium lacks sufficient thermal conductivity and may thus undergo an inside heat accumulation. Therefore, as shown in FIG. 1, beryllium is designed to be bonded to a pure copper or copper alloy as a heat conductive material.
In FIG. 1, reference numeral 1 denotes a pure beryllium, 2 a pure copper or copper alloy, and 3 a cooling pipe.
Conventionally, vacuum silver soldering has been mainly used as the bonding method though it is pointed out that this method may adversely affect plasma and that it lacks reliability in actual plant level.
That is to say, in the case of vacuum silver soldering, as silver is transformed into cadmium by nuclear reaction, it is highly likely that silver is mixed in plasma as impurities thereby adversely affecting it. Moreover, there may occur a chemical reaction between the solder material and bonded material; therefore, it is difficult to obtain achieve reliable bonding.
Thus, diffusion bonding method has been studied as a new bonding technique, though a satisfactory result has not yet been achieved.
That is to say, in the case of diffusion bonding, it is highly likely that intermetallic compounds are formed between pure beryllium and pure copper or copper alloy. Moreover, there is a significant difference in thermal expansion coefficient between pure beryllium and pure copper or copper alloy. In view of the above, there has been no effective solution for a problem of decreased bonding strength which is caused by thermal stress occurring during the bonding process or during operation of the nuclear fusion reactor.